It is known that communication by radio between a base station and a terminal, possibly a mobile terminal, is subject to phenomena that disturb the radio transmission between the antenna of the base station and the antenna of the mobile terminal, in particular to channel fadings due to destructive interference between signals which follow different propagation paths between the base station and the terminal.
The diversity of one of the features related to this transmission is one of the methods developed for alleviating fading. Thus, use is made of reception diversity, consisting in receiving the signal simultaneously on two antennas, transmission diversity, consisting in equipping the base station with several antennas transmitting the same signals, polarization diversity, frequency diversity (see for example the work “Réseaux GSM” [GSM networks] by X. Lagrange et al, published by Hermes Science Publications, 2000, page 161), etc.
It is known practice to use antennas comprising devices for altering the radiation pattern. Such adjustments pertain for example to the direction of transmission of the antenna or the width of the main transmission lobe.
These alterations of the radiation pattern may be mechanical, such as the orienting of an antenna arranged on an articulated support, mixed electrical/mechanical (cf. patent U.S. Pat. No. 6,198,458), or else purely electronic, as in FR-A-2 792 116.
Most antennas with electronic steering of the beam are composed of several antenna elements individually fed with signals obtained by phase shifting an initial signal. The value of the phase shift is determined as a function of the antenna element to which the phase-shifted signal is addressed, and the direction of transmission by the antenna results from the combining of the mutually phase-shifted signals transmitted by all the antenna elements.
Such antennas are sometimes used to focus a radio beam intended for a particular terminal. In a particular embodiment, the components of an up radio signal transmitted by the terminal and which are picked up by the various antenna elements, are analyzed in terms of phase shifts so as to estimate a direction in space from which this up signal originates. Corresponding phase shifts are then applied to the down signal intended for this terminal so that its transmission is oriented in this direction. Such electronic steering of the beam allows considerable reductions in interference level. It is used especially in satellite antennas. It has however been proposed that it be used in terrestrial systems in spite of its complex and expensive implementation.
An object of the present of the invention is to propose a novel radio diversity mode at the level of a base station.